Method of forming a leaded molded array package

ABSTRACT

In one embodiment, a method for forming a leaded molded array package includes placing a lead frame assembly into a molding apparatus having lead cavities. The method further includes forming seals between conductive leads within the lead frame assembly and the lead cavities, and encapsulating the lead frame assembly to form a molded array assembly. The molded array assembly is then separated into individual leaded molded packages.

The present application is a divisional application of U.S. applicationSer. No. 11/259,981 filed on Oct. 28, 2005, now U.S. Pat. No. 7,588,999.

FIELD OF THE INVENTION

This invention relates generally to electronic devices, and morespecifically to semiconductor packages and methods of assembly.

BACKGROUND OF THE INVENTION

The handheld consumer products market is aggressive in theminiaturization of portable electronics. Driven at the present time bythe cellular phone and digital assistant markets, manufacturers of thesedevices are challenged by ever shrinking formats and the demand for morePC-like functionality. This challenge asserts pressure on surface mountcomponent manufacturers to design their products to command the smallestarea possible. By doing so, this allows portable electronics designersto incorporate additional functions within a device without increasingthe overall product size.

In Chip Scale Packaging (CSP) technologies, manufacturers strive tobring the package size as close as possible to the size of thesemiconductor chip. The Quad Flat Pack No Lead (QFN) package is anexample of a widely accepted and used low cost chip scale package. TheQFN platform is cost effective because the platform uses highlyautomated assembly equipment and molded array package (MAP)encapsulation processes. However, because the QFN is leadless,manufacturers are unable to perform visual inspections of theconnections after the QFN devices are attached to next levels ofassembly such as printed circuit boards. This can result in end-productshaving reliability problems. Additionally, existing leaded CSPtechnologies such as the small outline IC (SOIC) platform are moreexpensive to manufacture than the QFN platform, and they are notcompatible with MAP assembly techniques.

Accordingly, a need exists for a structure and method of manufacturing aleaded package that is cost effective and compatible with MAP assemblytechniques.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a partial top view of a lead frame assembly inaccordance with the present invention at an intermediate step infabrication;

FIG. 2 illustrates a partial top view of the lead frame assembly of FIG.1 after subsequent processing to form a molded array assembly;

FIG. 3 illustrates a partial top view of a portion of the molded arrayassembly of FIG. 2 after further processing;

FIG. 4 illustrates a top view of a semiconductor package in accordancewith the present invention;

FIG. 5 illustrates a cross-sectional side view of the semiconductorpackage of FIG. 4;

FIG. 6 illustrates a cross-sectional side view of another semiconductorpackage in accordance with the present invention;

FIG. 7 illustrates a partial cross-section view of a manufacturingapparatus in accordance with the present invention for forming leadedmolded array packages;

FIG. 8 illustrates a partial top view of a portion of the manufacturingapparatus of FIG. 7;

FIG. 9 illustrates a partial cross-sectional end view of a portion ofthe manufacturing apparatus of FIG. 7 taken along reference line 9-9 inFIG. 8;

FIG. 10 illustrates a partial cross-sectional end view of a portion ofthe manufacturing apparatus of FIG. 9 at a subsequent step;

FIG. 11 illustrates a partial cross-sectional view of the manufacturingapparatus of FIG. 7 with an alternative lead frame assembly inaccordance with the present invention;

FIG. 12 illustrates a partial cross-sectional view of anothermanufacturing apparatus for forming molded packages in accordance withthe present invention;

FIG. 13 illustrates a partial top view of a portion of the manufacturingapparatus of FIG. 12;

FIG. 14 illustrates a partial bottom view of a portion of themanufacturing apparatus of FIG. 12; and

FIG. 15 illustrates a partial cross-sectional view of another portion ofthe manufacturing apparatus of FIG. 12.

DETAILED DESCRIPTION OF THE DRAWINGS

For ease of understanding, elements in the drawing figures are notnecessarily drawn to scale, and like element numbers are used whereappropriate throughout the various figures to denote the same or similarelements.

FIG. 1 shows a main lead frame assembly 10 in accordance with thepresent invention at an intermediate stage of manufacture. Lead frameassembly 10 includes a strip or array lead frame 11. In one embodiment,strip lead frame 11 is a substantially flat structure, and comprises asingle gauge chemically milled or stamped sheet of a material such as acopper alloy (e.g., TOMAC 4, TAMAC 5, 2ZFROFC, or CDA194), a copperplated iron/nickel alloy (e.g., copper plated Alloy 42), platedaluminum, plated plastic, or the like. Plated materials include copper,silver, or multi-layer plating such nickel-palladium and gold. Striplead frame 11 is configured, for example, to be compatible with standardQFN packaging assembly tools.

Strip lead frame 11 further includes elongated flags or die attachpaddles 12, which are integral with rails 14. In one embodiment, rails14 run perpendicular to the ends of flags 12, which are spaced apartfrom each other and substantially parallel with each other. Lead supportstructures or bars 16 are formed or placed in between elongated flags12, and include a plurality of conductive leads 17 placed in proximityto elongated flags 12. In one embodiment, conductive leads 17 and leadsupport bars 16 form a “comb-like” shape or structure with conductiveleads 17 substantially perpendicular with respect to elongated flags 12.

In one embodiment (shown for example, in FIG. 5), conductive leads 17and elongated flags 12 are not off-set with respect to each other, butare substantially co-planar or are as flat as practical. In anotherembodiment, they are offset with respect to each other, which is shownfor example in FIGS. 6 and 11. In another embodiment, conductive leads17 are rectangular in shape, but they may have other shapes compatiblewith selected printed circuit board or next level of assembly lay-outs.In a further embodiment, conductive leads 17 include bond posts, whichprovide increased surface area for connective structures.

Lead frame assembly 10 further includes a plurality of electronic chips18 and 19, which are attached to elongated flags 12 using an attachlayer 25 (shown in FIG. 5) such as a solder or a high conductivity epoxymaterial. By way of example, electronic chips 18 and 19 comprise powerMOSFETS, bipolar power transistors, insulated gate bipolar transistors,thyristors, diodes, sensors, passive devise, optical devices, integratedcircuits, logic devices, memory devices or combinations thereof. Asshown in FIG. 1, elongated flags 12 provide for electronic chips havingdifferent sizes, which provides increased manufacturing flexibility andsaves on costs because the same lead frame can be used to manufacturedifferent types of devices. In one embodiment, the pitch 107 betweenconductive leads 17 is constant.

Connective structures 21 electrically connect electronic chips 18 and 19to conductive leads 17. By way of example, connective structures 21include connective wires 210 (shown in FIG. 5), wire bonds, conductiveclips or straps 211 (shown in FIG. 5), or the like. When a clip or strapis used, a solder or conductive epoxy is suitable for attaching the clipor strap to lead 17 and electronic chips 18 and 19.

FIG. 2 shows a top view of main lead frame assembly 10 after portions ofthe structure are encapsulated with a plastic encapsulating layer 23. Inthe example shown, a strip molded array process is used whereencapsulating layer 23 is formed over elongated flags 12, connectivestructures 21, electronic chips 18 and 19, and portions of conductiveleads 17 in proximity to elongated flags 12. By way of example, assemblystructure 10 is placed in a molding apparatus such as a transfer moldingdevice. A sold resin pellet is placed in cull or pot 24. When pot 24 isheated to melt the solid resin pellet, the liquefied resin material isforced from pot 24 through runners 26 into the slot mold cavities toform a continuous encapsulating layer or encapsulating layer 23, whileleaving portions of leads 17 and lead support bars 16 exposed orun-encapsulated to form a molded array assembly 28.

FIG. 3 shows a top view of a portion of molded array assembly 28 duringa next step in fabrication. In this step, individual components aresingulated or separated from molded array assembly 28 using saw cutting,laser cutting, or similar techniques. By way of example, singulationlines 31 are first or horizontal cut lines, where the singulationapparatus cuts through encapsulating layer 23 and elongated flags 12.Singulations lines 32 are second or vertical cut lines, where thesingluation apparatus cuts through conductive leads 17 and rails 14.

FIG. 4 shows a top view of a semiconductor package 34 having exposed oroutwardly extending conductive leads 17 in accordance with the presentinvention. In one embodiment, conductive leads 17 extend outwardly fromthe encapsulated portion 23 from a lower surface 36 of package 34 asshown in the side cross-sectional view of package 34 in FIG. 5. Inaccordance with the present invention, package 36 includes molded sides38, which correspond to the sides where conductive leads extendoutwardly from and are formed with a molding structure duringencapsulation. Package 34 further includes singulated or cut sides 39(shown in FIG. 4) that are formed during the singulation, cutting orseparation process. Lower surfaces 170 and 120 of conductive leads 17and elongated flag 12 are exposed as shown for coupling or connecting toa next level of assembly.

FIG. 6 shows a side cross-sectional view of an alternative package 44 inaccordance with the present invention. Package 44 is similar to package34 except that elongated flag 12 is off-set with respect to conductiveleads 17 so that encapsulating layer 23 covers lower surface 120 ofelongated flag 12. Additionally, conductive leads 17 are shown after anoptional lead shaping process that bends them into a desired shape suchas a “gull-wing” shape as shown. Lower surfaces 170 of conductive leads17 are exposed as shown in FIG. 6.

In accordance with the present invention, strip lead frame 11 isprovided without, free of, or independent of epoxy mold dam-bars toavoid having to use expensive punch tools as part of the assemblyprocess. However, a method and molding structure were needed to formpackages 34 and 44 without forming encapsulating material or flashbetween conductive leads 17 during the encapsulating process. FIGS. 7-11show a process and structure in accordance with the present inventionfor manufacturing leaded packages such as, but not limited to, packages34 and 44 with dam-bar free lead frames.

FIG. 7 shows a cross-sectional and side view of a molding apparatus 71for forming leaded molded array packages in accordance with the presentinvention. In one embodiment, apparatus 71 is used to slot mold orencapsulate lead frame assembly 10 as shown in FIG. 2. Apparatus 71includes a top mold die 72 and a bottom mold die 73, which has a cavity76. In this embodiment, lead frame assembly 10 is positioned in bottommold die 73 such that a portion 177 of lead frame 11 extends above theupper surface or plane 75 of bottom mold die 73. As will be explainedmore fully below, this is accomplished using, for example, tapered,shaped or narrowed lead cavities. Additionally, in this embodiment,electronic chip 18 is placed in a die down configuration, which placeselectronic chip 18 and connective structures 21 within cavity 76.

Bottom mold die 73 further includes a plurality of semi-recessed leadcavities, channels or portions 77 for supporting conductive leads 17. Inaccordance with the present invention, channels 77 are narrowed, shapedor tapered so that portions 177 of conductive leads 77 initially extendabove major surface 75. As a result, when top mold die 72 is broughtinto contact bottom mold die 73, the soft or pliable portions of leads17 are crimped into lead cavities 77. This forms a seal that preventsthe flow of encapsulating material 23 between conductive leads 17 duringthe molding process.

FIG. 8 is a top view of a portion of bottom mold die 73, which shows alead cavity 77 and a conductive lead 17 placed within lead cavity 77. Inaccordance with the present invention, lead cavity 77 is tapered,narrowed or has a shape that forms a seal 81 with conductive lead 17 toprevent the flow of encapsulating material 23 between conductive leads17. By way of example, lead cavity 77 is shown with tapered or narrowedportions 772 that have widths less than widths of conductive leads 17.Lead cavity 77 further includes, for example, widened portions 771,which are wider than conductive leads 17 as shown.

FIG. 9 is a partial cross-sectional end view of apparatus 71 taken alongreference line 9-9 of FIG. 8, which shows conductive lead 17 withportion 177 above major surface 75 of bottom mold die 73. FIG. 9 furthershows lead cavity 77 with an example of a shape that has taperedportions 772 that subsequently forms a seal 81 with a conductive lead17.

FIG. 10 shows apparatus 71 from the view of FIG. 9 after top mold die 72is placed in contact with bottom mold die 73, which crimps portions ofconductive leads 17 into lead cavities 77 to form seals 81. That is,molding apparatus 71 is closed thereby forcing conductive leads 17 intolead cavities 77 to form seals 81 between conductive leads 17 and leadcavities 77. This is achieved for example, by choosing a soft materialfor conductive leads 17. In other words, conductive leads 17 comprise amaterial having a hardness less than that of top mold die 72 and bottommold die 73. In accordance with the present invention, this acts toprevent the flow of encapsulating material between conductive leads 17and eliminates the need for epoxy mold dam-bars on lead frame 11. Thissaves on material costs and eliminates the need for using expensivepunch tools to remove the dam-bars after molding.

FIG. 11 is a cross-sectional view of apparatus 71 with an alternativelead frame 111. Lead frame 111 is similar to lead frame 11 except thatflags 12 are offset a distance 110 from conductive leads 17. Offset 110provides, for example, package structure 44 shown in FIG. 6 where flag12 is encapsulated or not exposed.

Turning now to FIGS. 12-15, a structure and method is described forforming single cavity molded packages with leads on more than two sidesof the packages without using lead frames with epoxy mold dam-bars inaccordance with present invention. FIG. 12 shows a cross-sectional viewof a portion of a molding apparatus 271 that includes a top mold die272, a bottom mold die 273, and a mold plate 274. In one embodiment,bottom mold die 273 has a single cavity 276 for holding a lead framearray 281. Lead frame array 281 includes a plurality of flags 212 forsupporting electronic chips 289 and a plurality of conductive leads 217in proximity to flags 212. Electronic chips 289 are electronicallycoupled to conductive leads 217 using connective structures 221 such asconnective wires, clips, straps or the like.

Top mold die 272 is shown with an opening or aperture 372 for injectinga resin mold compound or molding material into apparatus 271. Mold plate274 includes a cavity 376 that is shaped according to a desired outlineor shape of the finished product. Mold plate 274 further includes anopening or aperture 374 for allowing the resin mold compound to flowinto cavity 376. Mold plate 274 also includes a plurality of leadcavities, cut-outs, or channels 377, which are shaped, narrowed ortapered to form a seal or to crimp-off a portion of conductive leads 217during the molding process.

FIG. 13 shows a partial top view of cavity 276 formed in bottom mold die273 for holding lead frame array 281 having a plurality of flags 212, aplurality of conductive leads 217, and a plurality of electronic chips289 as described in conjunction with FIG. 12. Dashed lines 131represents an example of an outline for an encapsulation layer or moldedpassivation layer corresponding to the shape of cavity 376 of mold plate274.

FIG. 14 is a partial bottom view of mold plate 274 showing a lead cavity377 with a conductive lead 217 placed, sealed, or crimped therein. Leadcavity 377 has for example, tapered portions 872 that form a seal 881with conductive lead 217 when contact is made between mold plate 274 andconductive leads 217. Conductive leads 217 comprise a soft material suchas copper or the like or a material having a hardness less than thematerial that mold plate 274 comprises, and are crimped or forced intotapered portions 872 to form seals 881 between conductive leads 217 andlead cavities 377 as shown when molding apparatus 271 is closed.

FIG. 15 shows a partial cross-sectional view along reference line 15-15in FIG. 14 with the addition of top mold die 272 and bottom mold die273, and shows an example of conductive lead 217 crimped within leadcavity 377 to form seal 881. This prevents resin mold compound fromflowing between conductive leads 217 and eliminates the need for epoxymold dam-bars.

Thus, it is apparent that there has been provided, in accordance withthe present invention, a structure and method for forming leaded moldedarray packages using lead frames independent of epoxy mold dam-bars. Themethod and structure provide leaded molded array packages that are costeffective and that provide manufacturers, among other things, with thedesired ability to inspect bond integrity when the packages are attachedto a next level of assembly.

Although the invention has been described and illustrated with referenceto specific embodiments thereof, it is not intended that the inventionbe limited to these illustrative embodiments.

1. A method for forming leaded molded packages comprising the steps of:placing a lead frame assembly having conductive leads, flag portionswith electronic chips attached thereto and electrically coupled to theconductive leads into a molding apparatus having a plurality of leadchannels shaped to prevent flow of encapsulating material betweenadjacent conductive leads, wherein the lead frame assembly is free ofdam-bars; crimping the conductive leads within the lead channels;molding portions of the lead frame assembly with an encapsulating layerwhile leaving portions of the conductive leads exposed to form a moldedlead frame assembly; and singulating the molded lead frame assembly intoindividual leaded molded packages wherein the step of crimping includesthe steps of: placing the conductive leads into the lead channels havingtapered portions with widths less than widths of the conductive leads;and closing the molding apparatus to force the conductive leads into thetapered portions thereby forming seals between the conductive leads andthe lead channels.
 2. The method of claim 1, wherein the step of placingthe lead frame assembly includes placing a lead frame assembly havingelectronic chips electrically coupled to the conductive leads withconductive straps.
 3. The method of claim 1, wherein the step of placingthe lead frame assembly includes placing a lead frame assembly havingelectronic chips electrically coupled to the conductive leads withconductive straps and wire bonds.
 4. The method of claim 1, wherein thestep of providing the lead frame assembly includes providing a leadframe assembly having conductive leads on all sides of at least oneelectronic chip.
 5. The method of claim 1, wherein the step ofsingulating includes separating the molded lead frame assembly throughportions of the encapsulating layer and the flag portions and separatingthrough the conductive leads.
 6. The method of claim 5, wherein the stepof separating the molded lead frame assembly through portions of theencapsulating layer and the flag portions includes separating in a firstdirection, and wherein the step of separating through the conductiveleads includes separating through the conductive leads in a seconddirection substantially perpendicular to the first direction.
 7. Themethod of claim 1, wherein the step of providing the lead frame assemblyincludes providing the lead frame assembly, wherein the flag portionsand the conductive leads are substantially co-planar.
 8. The method ofclaim 1, wherein the step of providing the lead frame assembly includesproviding the lead frame assembly wherein the flag portions are offset adistance from the conductive leads.
 9. The method of claim 1, whereinthe step of placing the lead frame assembly into the molding apparatusincludes placing the lead frame assembly into a molding apparatus havinga bottom mold die with a cavity and the plurality of lead channels,wherein the placing step further includes placing the lead frameassembly into the bottom mold die so that the electronic chips arewithin the cavity of the bottom mold die.
 10. The method of claim 1,wherein the step of placing the lead frame assembly into the moldingapparatus includes placing the lead frame assembly into a moldingapparatus having a bottom mold die having a single bottom cavity and amold plate having a mold cavity and the plurality of lead channels, andwherein the placing step further includes placing the lead frameassembly so that an electronic chip is within the mold cavity.
 11. Themethod of claim 10, wherein the step of placing the lead frame assemblyincludes placing a lead frame assembly having conductive leads on allsides of the electronic chip.
 12. The method of claim 1, furthercomprising the step of forming the conductive leads to provide a desiredshape.
 13. The method of claim 1, wherein the step of placing the leadframe assembly includes placing a lead frame assembly having electronicchips of different sizes.
 14. The method of claim 1, wherein the step ofmolding includes molding portions of the lead frame assembly whileleaving surfaces of the flag portions exposed through the encapsulatinglayer.
 15. The method of claim 1, wherein the step of molding includesmolding portions of the lead frame assembly so that the encapsulatinglayer covers surfaces of the flag portions opposite to the electronicchips.
 16. The method of claim 1, wherein the step of placing the leadframe assembly includes placing a lead frame assembly having railsrunning substantially perpendicular to and integral with the flagportions.
 17. The method of claim 16, wherein the step of placing thelead frame assembly includes placing a lead frame assembly having leadsupport structures including the conductive leads, and wherein theconductive leads are substantially perpendicular to the flag portions.